Properties of ASA/SAN/NBR ternary blends: effect of AN content in NBR

Abstract

Nitrile–butadiene rubbers (NBRs) with different acrylonitrile (AN) contents were used to toughen acrylonitrile–styrene–acrylic terpolymer/styrene–acrylonitrile copolymer (ASA/SAN) blends. The properties of the ASA/SAN/NBR ternary blends were investigated via dynamic mechanical analysis, heat distortion temperature, Fourier transform infrared spectroscopy and scanning electron microscopy (SEM). The effects of AN content in NBR on physical properties, heat resistance and morphology of the ternary blends were studied. Heat distortion temperature of the blends decreased with increasing AN content of NBR. The impact strength reached the maximum value when 20 phr NBR with 26 wt-%AN content was added. Images (SEM) were in accordance with results of mechanical properties.

Keywords

ASA SAN NBR Toughness AN content

Introduction

As an impact resistance thermoplastic, acrylonitrile–styrene–acrylic (ASA) was developed in the 1970s and was widely used in many fields due to its excellent properties, such as good toughness, colour and dimensional stability, and thermal stability. Moreover, it is a trend that ASA will finally be replaced by acrylonitrile–butadiene–styrene (ABS) because of the outstanding weatherability of ASA compared with that of ABS.

Mixing different polymers has revealed a new realm of technically important materials. Their properties can be altered by varying the composition of polymer blends, and the compatibility between the different polymer components is the key factor. As compatibility is one of the most important factors considered in polymer alloys, ASA/styrene–acrylonitrile (SAN) blend system was investigated in our previous work. ASA have a good interfacial adhesion with SAN, since it is a core–shell structure polymer that contains a grafted SAN shell. However, incorporation of SAN decreased the toughness of the material, and the impact strength of ASA/SAN (30/70) decreased to 1·0 kJ m⁻². Acrylic resin (ACR) was used to toughen the ASA/SAN (30/70) blend system in our previous work. However, the impact strength of ASA/SAN/ACR (30/70/30) is still low (6·9 kJ m⁻²), which is relatively low for industrial application.¹ Therefore, the enhancement of toughness of the blend is necessary.

Toughening brittle polymer by dispersed rubber particle is a typical example of application in polymer blends. Acrylonitrile–butadiene rubber (NBR) modified blends have received a lot of interest because of their oil resistance property and excellent mechanical properties. For instance, poly(vinyl chloride)/NBR,² SAN/NBR,³ polyethylene/NBR,⁴ polyamide/NBR,⁵ ethylene vinyl acetate copolymer/NBR,⁶ polypropylene/NBR⁷ and poly(ethylene terephalate)/NBR⁸ were reported previously. Most of these studies involved mechanical properties, morphology and thermal stability. The miscibility of the blends, as well as the dispersion of rubber particles and the acrylonitrile (AN) content of NBR, was investigated.

NBR is an important modifier for SAN or ABS to improve the impact strength of mat surface appearance.⁹ Considering that both ASA and ABS have the SAN segment in their macromolecular structure, NBR is considered to be a competitive candidate to improve the toughness of ASA/SAN (30/70) blend.

In random copolymer blends, both the miscibility and mechanical properties depend upon the specific composition of a certain copolymer in the blends.⁹ The previous literatures reported that AN content of NBR affected the compatibility and properties of NBR contained blends.¹⁰

The aim of the present work is to investigate the toughening efficiency of the several kinds of NBRs with different AN contents. ASA/SAN/NBR ternary blends with different composition were prepared via melt blending; mechanical properties, glass transition behaviour, heat resistance and morphology were discussed in detail.

Experimental

Materials

ASA (Luran S 776 SE) was supplied by BASF Company Ltd (Korea). SAN (D-178) was supplied by Zhenjiang GPPC Chemical Co. Ltd (China). The NBR with different AN contents was supplied by JSR Corporation (Japan) and the details were listed in Table 1.

Table 1

Characteristics of NBR used in this study

Resin notation	Grade	AN content/wt-%
NBR1	N 250S	20
NBR2	N 240S	26
NBR3	N 230S	35
NBR4	N 220S	41

The ASA/SAN/NBR ternary blends were prepared by adding 10 and 20 phr NBR with different AN contents to ASA/SAN (30/70) blends. In order to clarify toughening efficiency of different NBRs, each ASA/SAN/NBR ternary blend has an experimental name, and the details were listed in Table 2.

Table 2

Experimental name of each ASA/SAN/NBR ternary blend

Experimental name	Polymer blends
M0(N0)	ASA/SAN/NBR (30/70/0)
M1	ASA/SAN/NBR1 (30/70/10)
M2	ASA/SAN/NBR2 (30/70/10)
M3	ASA/SAN/NBR3 (30/70/10)
M4	ASA/SAN/NBR4 (30/70/10)
N1	ASA/SAN/NBR1 (30/70/20)
N2	ASA/SAN/NBR2 (30/70/20)
N3	ASA/SAN/NBR3 (30/70/20)
N4	ASA/SAN/NBR4 (30/70/20)

Sample preparation

There are two widely used methods to modify materials in industry. Materials are firstly mixed by high speed mixer, after which the mixture is granulated by two-screw extruder (step 1). Materials are blended by an inner mixer or two-roll mill, after which the granules are produced by platen pelletiser or single screw extruder (step 2). The first method is applied because of its high automation and efficiency. However, the original materials used in step 1 must be provided as powders or granules. The rubber used in this work is bulk rubber, which is inappropriate for blending by a two-screw extruder. Therefore, the two-roll mill was adopted. The blended samples were subsequently prepared by compression moulding. Acrylonitrile–styrene–acrylic terpolymer, SAN and NBR were mixed by a two-roll mill at 180°C. After that, a flat plate vulcanisation machine was used to compression mould the mixed blends into sheets of 2 and 4 mm thickness at 180°C under a 10 MPa pressure. The dumb bell shaped pieces cut from 2 mm sheets were used for tensile tests, and rectangular samples (80×10×4 mm) cut from 4 mm sheets were used for flexural, impact and heat distortion temperature (HDT) tests.

Characterisation

Glass transition temperature

Glass transition temperature of ASA/SAN/NBR ternary blends was determined by a modular compact rheometer (MCR 302; AntonPaar, AUT). The dynamic mechanical analysis (DMA) of the blends was carried out in a torsion mode with a frequency of 1 Hz in the temperature range from −90 to 150°C at a heating rate of 3°C min⁻¹. The dimension of the test regular specimens was measured before the tests, and T_g was defined as the peak of tan δ.

Heat distortion temperature

HDT of the prepared ASA/SAN/NBR ternary blends was evaluated using a Vicat/HDT equipment (ZWK1302-2; Shenzhen SANS Testing Machine Co., China). All the tests were conducted under the maximum blending stress of 1·80 and 0·45 MPa with a heating rate of 120°C h⁻¹, following ISO 75-2.

Fourier transform infrared spectroscopy analysis

The interactions among components of ASA/SAN/NBR ternary blends were characterised by Fourier transform infrared (FTIR). The spectra of the samples (<30 μm) were obtained on an FTIR spectrometer (Nicolet iS5; Thermo Fisher, USA) with a resolution of 4 cm⁻¹ in the wave number range of 4000–400 cm⁻¹.

Melt flowrate

A melt flow indexer (XNR-400A; Changchun Second Factory, China) was used to evaluate the processability of ASA/SAN/NBR ternary blends in this work. The tests were conducted at 220°C under a 10 kg load, according to ISO 1133.

Mechanical properties

Toughening effect of NBR with different AN contents to ASA/SAN binary blends was evaluated by the test of Notched Izod impact strength, which was carried out on an Izod impact tester (UJ-4; Chengde Machine Factory, China) at room temperature following ISO 180. Tensile and flexural tests were conducted on a universal testing machine (CMT 5254; Shenzhen SANS Testing Machine Co. Ltd, China) with an invariant rate of 5 and 2 mm min⁻¹, according to ISO 527 and ISO 178 respectively. Hardness was tested on an XHS-D hardness tester (Yingkou Material Testing Machine Co. Ltd, China) according to ISO 868.

Scanning electron microscopy analysis

A scanning electron microscope (JSM-5900; JEOL, Japan) with an accelerating voltage of 15 kV was used to observe the micrographs of the samples. The fractured surface was coated with a thin conductive layer of sputtered gold before viewing.

Results and discussion

Glass transition temperature

Dynamic mechanical analysis was used to characterise glass transition behaviour of the ternary blends, and tan δ curves were exhibited in Fig. 1. The results suggested that all the NBR contained blends exhibited two T_gs in low temperature region and one in high temperature region, and all the values were listed in Table 3. In ASA/SAN (30/70) blends, the T_g at −53·1°C is assigned to polyacrylate of ASA, and 113·8°C is the SAN segment in the blends. The data in Table 3 revealed that all the M and N series blends showed a T_g of around −50°C, which is almost unchanged. Thus, the other T_g observed in low temperature region is attributed to the transition of NBR rubber. It is interesting to note that T_g,3 of all the blends appears almost at the same temperature except M4 and N4: the T_g of M4 and N4 decreased to 109·1 and 107·1°C respectively. The results show that in M4 and N4, some amount of NBR is preferentially dissolved into SAN rich phase, and the decrease in the T_g was observed,⁹ indicating that the more NBR added, the lower T_g,3 was obtained.

Tan δ curves of ASA/SAN/NBR ternary blends

Table 3

T_g values of ASA/SAN/NBR blends with different components

Sample	T_g,1/°C	T_g,2/°C	T_g,3/°C	Sample	T_g,1/°C	T_g,2/°C	T_g,3/°C
M0	−53·1	…	113·8	N0	−53·1	…	113·8
M1	−65·6	−50·7	115·7	N1	−64·5	−49·3	113·6
M2	−48·6	−39·3	115·6	N2	−46·8	−35·4	115·8
M3	−49·2	−35·5	114·0	N3	−47·6	−32·1	114·1
M4	−50·5	−12·7	109·1	N4	−50·9	−11·8	107·1

Heat distortion temperature

Figure 2 shows HDT of ASA/SAN/NBR ternary blends, which was used to evaluate the heat resistance of prepared blends.

Heat distortion temperatures of ASA/SAN/NBR ternary blends

Generally speaking, HDT decreased slightly with the increasing NBR content, i.e. HDT of ASA/SAN/NBR (30/70/10) is almost the same as that of ASA/SAN/NBR (30/70/20) except the NBR4 contained blend. It can be seen that HDT of the blends decreased as AN content increased, and this trend is independent with the dosage of NBR: blends containing 10 and 20 phr NBR exhibit the same trend. It is interesting to notice that HDT of M1 and N1 measured under the maximum pressure of both 1·80 and 0·45 MPa is almost the same as that of M0 (N0). Two reasons explain for this phenomenon: one is the excellent heat resistance of the matrix, and the other is about the compatibility of the blend system. In our previous work, heat resistance of ASA/SAN binary blends was evaluated, and the results showed that values of HDT increased with SAN content, because SAN is a resin with relatively high stiffness.¹¹ Since ASA/SAN (30/70) binary blends showed a good resistance, the addition of NBR could not affect the heat resistance of the blends so much. With respect to the little decrease in HDT with the increasing AN content in NBR, it is the matter about miscibility: the increase in AN content enhances the partial miscibility between SAN and NBR; as a result, T_g,SAN is reduced by the partial dissolution of NBR molecules into SAN matrix.⁹

FTIR spectra

FTIR analysis was used to characterise the interactions among components of the blends, and Fig. 3 shows the FTIR spectra of ASA/SAN/NBR ternary blends in this work. The prominent infrared bands were listed in Table 4.

FTIR spectra of ASA/SAN/NBR ternary blends

Table 4

Observed frequencies of ASA/SAN/NBR ternary blends

Bands/cm⁻¹	Assignment
1735	Stretching vibration of carbonyl (C = O)¹²
2238	Stretching vibration of cyano (C≡N)¹³
701	Characteristic peaks of single substituted phenyl ring¹³
1602	Characteristic peaks of vibration of benzene skeletal ring¹⁴
2932, 1453	C–H asymmetric stretching and bending vibration of >CH₂¹²
2874, 1373	C–H symmetric deformation of –CH₃¹⁵
1162	C–C( = O)–O stretching¹⁶
970	= CH₂ stretching vibration¹⁷

As can be seen in Fig. 3, the spectra of different blends were almost the same but showed some differences. The characteristic peaks at 1735, 2238, 701, 1602, 2932, 1435, 2874, 1373 and 1162 cm⁻¹ showed no obvious shift, but there was variation band shown at 970 cm⁻¹: when NBR was added into ASA/SAN (30/70) binary blends, the FTIR spectra exhibited the characteristic peaks of = CH₂ group contained.

With the results analysed above, since no obvious shift of wave number of the characteristic peaks was observed, there existed no strong specific intermolecular interactions in the ternary blends prepared in this work. Therefore, the addition of NBR to ASA/SAN binary blends is a physical process.

Melt flowrate

The melt flowrate (MFR) is an important characteristic of polymer processing properties. In this work, MFR of the blends was determined at 220°C under a 10 kg load, and the results were displayed in Fig. 4.

Melt flowrate of ASA/SAN/NBR ternary blends

The results showed that addition of NBR decreased values of MFR, and the more NBR added, the lower MFR was detected. The MFR reduction caused by the incorporation of NBR coincided with some references: the addition of NBR increased the rubber content of the blends, and accordingly, the viscosity increased. Since MFR is inversely proportional to the viscosity, the value decreased with the NBR content. When comparing MFR of blends with different components, MFR increased with AN content of NBR added, and the results are consistent with that of NBR contained blends reported in early literatures.¹⁸

Mechanical properties

Mechanical properties of ASA/SAN/NBR ternary blends were displayed in Figs. 5–8.

Impact strength of ASA/SAN/NBR ternary blends

Tensile properties of ASA/SAN/NBR ternary blends

Flexural properties of ASA/SAN/NBR ternary blends

Hardness of ASA/SAN/NBR ternary blends

Figure 5 shows the impact strength of ASA/SAN/NBR ternary blends, and the results suggested that the values of the impact strength of ASA/SAN/NBR (30/70/10) blends were almost the same as those of ASA/SAN/NBR (30/70/0) blend: 10 phr NBR used in this work was not enough to improve the toughness of ASA/SAN (30/70) binary blend significantly. However, the impact strength of ASA/SAN/NBR (30/70/20) ternary blends showed some differences: the values of impact strength of N3 and N4 are both ∼2·0 kJ m⁻², while N1 is 14·4 kJ m⁻² and N2 is 18·8 kJ m⁻². The results of impact strength showed that NBR1 and NBR2 can enhance toughness in a relative large scale, and the AN content of NBR has a great influence on the toughening efficiency. The results above exhibited that when AN content is <30 wt-%, NBR can significantly enhance toughness of ASA/SAN (30/70) blends, but when AN content is higher than 30 wt-%, the toughening effect is not ideal.

In rubber toughened polymers, the dispersed rubber phase enhanced the toughness mainly by promoting the energy dissipation.⁹ As reported in early literature, matrix crazing and rubber cavitation were the major toughening mechanism of ABS.¹⁹ Usually, crazing is more likely to be initiated at large rubber particles and cavitation is more likely to occur at small rubber particles. The NBR incorporated with different AN contents caused the different rubber particles, resulting in the different toughening efficiency, since there existed an optimum rubber diameter reported in SAN/rubber blends.²⁰ The impact strength had the largest values when the AN content in NBR was 34 wt-% in SAN/NBR blend system,⁹ but in this work, the impact strength reached the maximum when AN content is 26 wt-%. The reason may be that toughening is a complicated process where many variable factors can alter the optimum value, but the changing tendency fitted with that of early reported.

As shown in Fig. 6, tensile strength of the blends decreased when NBR was added, and values of ASA/SAN/NBR blends with four different NBRs showed no great difference. Tensile strength of ASA/SAN/NBR (30/70/10) blends is ∼50 MPa, and when 20 phr NBR was added, the value decreased to ∼40 MPa. In other words, incorporation of the rubber phase decreases tensile strength of the blends in comparison with pure ASA/SAN (30/70) blends. Furthermore, the more NBR added, the lower tensile strength was obtained. AN content of NBR seems to be unable to affect the tensile properties significantly. Elongation at break values of the blends were not enhanced so much with the incorporation of different NBRs. The values exhibited no great variation, and elongation at break of M4 and N4 is 13 and 18%.

Figure 7 shows the flexural properties of ASA/SAN/NBR ternary blends. As can be seen from the results, incorporation of NBR decreased both flexural strength and flexural modulus. The flexural strength of M2, M3 and M4 was almost the same (∼78·0 MPa), while the flexural strength of M1 decreased to 71·5 MPa. When 20 phr NBRs were added, the flexural strength increased from 53·3 MPa (N1) to 65·1 MPa (N4). With respect to flexural modulus, the values of ASA/SAN/NBR (30/70/10) blends varied between 2100 and 2400 MPa, and the constant interval of N series blends’ flexural modulus is 1800–1950 MPa. Flexural properties are usually used to characterise stiffness of the materials, and the decline of flexural strength indicated the reduction of stiffness of the blends.

Results of hardness of the blends (Fig. 8) revealed that hardness of the blends increased as the AN content of NBR increased. In both M and N series blends, the hardness of NBR1 contained blends was lower than that of other blends, and the hardness of the other blends is almost the same, i.e. NBR with 20 wt-%AN content obviously decreased hardness of the blends, but others did not.

The results of mechanical properties suggested that 10 phr NBR cannot enhance the toughness of the ASA/SAN (30/70) blends, and when 20 phr NBR was added, NBR1 and NBR2 can enhance the toughness by a relative large scale. Considering tensile and flexural properties, blend with 20 phr NBR2 (AN content is 26 wt-%) exhibits the best mechanical properties.

Scanning electron microscopy analysis

Images (SEM) displayed in Fig. 9 were used to observe the microstructure of ASA/SAN/NBR ternary blends. The images were taken from the impact fracture surface of blends with different components. As can be seen from the SEM images, the ASA/SAN/NBR blend system showed good compatibility despite the different kinds of NBR. Comparing the nine images in Fig. 9, we can find that most of the impact fracture surface was flat and clear cut, indicating the poor toughness. When 10 phr NBR was added, the fracture surface of the blend is tougher than that of pure ASA/SAN (30/70) blends: toughness enhanced to some extent, but not so much. It is obvious to note that N1 and N2 exhibited remarkable difference in morphology. In addition, the rough fracture surface with thread-like morphology indicated the significant enhancement of toughness of the blends, which was in great agreement with the results of mechanical properties.

Images (SEM) of ASA/SAN/NBR ternary blends

Conclusions

NBR with different AN contents was added to toughen ASA/SAN (30/70) blend, and properties were investigated to compare the toughening efficiency of different kinds of NBR. The results of DMA showed that T_gs of the two rubber phase were observed and did not exhibit obvious variations. Obvious shift of characteristic peaks was not observed in FTIR spectra, indicating that it is the physical action during the preparation of the blends. Heat resistance of the blends decreased with the increasing AN content, but not so much. Mechanical properties suggested that the toughness of the blend reached the maximum when 20 phr NBR that has 26 wt-%AN content was added. SEM photographs showed that the blends exhibited good compatibility, and the fracture morphology fit well with the results of mechanical properties.

Footnotes

Acknowledgements

This research was supported by the Innovation Foundation for Graduate Students of Jiangsu Province (grant no. CXLX12_0421) and the Priority Academic Program Development of Jiangsu Higher Education Institutions.

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